The present invention provides methods for implanting ions in a target substrate. More particularly, the invention provides methods for producing a buried insulating layer, which is substantially free of defects, in a semiconductor substrate.
Ion implantation techniques are useful in forming a class of structures, known as silicon-on-insulator (SOI) structures or semiconductor-on-insulator structure, that have an insulating layer beneath a thin surface silicon or other semiconductor film. SOI structures are of value in construction of semiconductor devices, micro-electro-mechanical systems (MEMS), and optical devices provided that the density of defects in the material is sufficiently low, and the cost of manufacturing the material is acceptable for the intended application.
Implantation of oxygen ions into a silicon wafer to form a buried insulation layer is one method for SOI construction, and is known by the acronym SIMOX (separation by implanted oxygen). It is desirable to decrease the required dose of oxygen ions in a SIMOX process to satisfy certain requirements imposed by device physics, and to decrease the fabrication cost of SOI wafers. However, non-optimized implantation doses can result in creation of defects in the buried oxide layer that lead to poor electrical quality of SOI structures.
A number of approaches have been suggested in the art to address this problem. Some of these approaches include, for example, the use of a pre-amorphization implant or the use of an internal thermal oxidation step in the process of producing an SOI structure. These approaches typically suffer from a number of limitations. Some of these approaches impose severe constraints on the thickness of the insulating layer and/or the thickness of the overlying silicon layer. Moreover, the manufacturing process associated with these approaches can be cumbersome, difficult to control, and costly.
Accordingly, there exists a need for an improved method for production of SOI structures where the buried oxide layer is substantially free of silicon inclusions.
There exists also a need for an improved method for production of SOI structures which can be readily implemented and is cost efficient.
The present invention provides a method for producing a buried insulating layer in a semiconductor substrate. The method includes multiple steps of implantation of oxygen ions in a substrate, where each implantation step is followed by an annealing protocol. The substrate is pre-heated before each implantation step to a temperature in a range of 400 to 600xc2x0 C. A incremental sub-stoichiometric dose of oxygen ions is implanted in the substrate during a first implantation step by exposing the pre-heated substrate to a beam of oxygen ions having an energy in a range of approximately 40 to 210 keV. More preferably, the energy of the oxygen ions is selected to be in a range of about 120 to 145 keV, or alternatively in a range of about 165 to 190 keV. The current of the oxygen beam is preferably selected to be approximately 45 mA to maintain the temperature of the substrate at about 450 to 700xc2x0 C. in combination with lamp heating.
Subsequent to the first implantation step, the substrate is subjected to an annealing protocol in an inert ambient. An inert ambient as used herein contains a non-oxidizing gas as its main constituent. The inert ambient can have trace amounts of oxygen, e.g., an atmosphere of argon or nitrogen having less than about 1 percent of oxygen. The annealing protocol is typically performed over a period of a few hours and at a number of elevated temperatures. For example, an annealing step can begin at a push temperature of about 800xc2x0 C., followed by a temperature ramp at a rate of approximately 5-10xc2x0 C./minute to increase the temperature of the substrate to about 1000xc2x0 C. Subsequently, the temperature can be increased at a rate of about 2xc2x0 C./minute to approximately 1300xc2x0 C. The substrate is held at this temperature for a few hours. Subsequently, the temperature is ramped down to 1000xc2x0 C. at a rate of, for example, 1-2xc2x0 C./minute and is further ramped down at a rate of, for example, 5-10xc2x0 C./minute to about 800xc2x0 C. The substrate is then pulled and allowed to cool to room temperature (e.g., 20xc2x0 C.).
A second sub-stoichiometric dose of oxygen, preferably less than the dose in the first implantation step, is then implanted in the substrate by exposing the substrate to oxygen ions having an energy greater than the energy of the ions in the first implantation step. A ratio of the second dose to the first dose is preferably in a range between 0.2 to 0.9, and the energy of the oxygen ions during the second implantation step is higher than their energy during the first implantation step by a value in a range of approximately 5 to 75 keV.
After the second implantation step, the substrate is subjected to a second annealing protocol preferably in an inert ambient atmosphere that is different from the atmosphere in which the first annealing protocol is performed. For example, if the first annealing protocol is performed in an argon atmosphere, the second annealing protocol can be performed in a nitrogen atmosphere. Further, the maximum temperature of the substrate during the second annealing protocol is preferably selected to be higher than the maximum temperature in the first annealing protocol.
An oxidation step can be optionally performed during at least a portion of one of the annealing steps. For example, during the first annealing step when the temperature of the substrate is about 1000xc2x0 C., the substrate can be subjected to an oxidation step by increasing the oxygen content of the ambient atmosphere to a value in a range of about 1 to 100 percent. Subsequent to the oxidation step, the inert ambient atmosphere can be restored and the annealing step can continue, as described above.
In another aspect of the invention, a capping layer of SiO2 can be deposited on the substrate after the second implantation step and before the second annealing step by methods known in the art, for example, by deposition from a TEOS source. The capping layer advantageously allows employing the same or different atmospheres during the different annealing steps.
Illustrative embodiments of the invention will be described below with reference to the following drawings.